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Thread: Chemistry Challenge Involving Water

  1. #1 Chemistry Challenge Involving Water 
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    Oct 2009
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    Hello,

    I have been working on an idea for a class room model that would illustrate 2 topics of education: water cycles and liquid properties. The goal would be to place water in a chamber or series of chambers and apply a vacuum to lower the boiling temperature. A heat source less than 212 degrees F would be applied to boil the water and show the relationship between pressure and temperature and water phases. The water vapor would then condense and return to a liquid form and begin the cycle again demonstrating the water cycle. Research on this subject leaves me with two questions that I have been stuck on for over a week:

    1. All examples I have found show applying a continuous vacuum to the apparatus in order to keep the pressure low when water vapor is being created. Is there any way to create a vacuum chamber and then ‘cap’ the system so it is self-contained and does not require leaving a vacuum pump running? What if the initial pressure was so low that the water vapor created did not increase the pressure significantly? What about mechanically expanding or compressing the chamber?

    2. I have read about ‘de-gassing’ a liquid in a vacuum and my understanding of this is when the soluble air particles in the water are pulled out. This creates the appearance of boiling but it is deceptive and only lasts about 60 seconds. Once this stops, if the temperature is raised and the pressure is lowered will the water ‘boil’ or does degassing prevent boiling from happening?

    I appreciate all of your help on this subject. Chemistry was never my strong point in school and I am attempting to work with what little knowledge I have.


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  3. #2 Re: Chemistry Challenge Involving Water 
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    Quote Originally Posted by engineerjoe

    1. All examples I have found show applying a continuous vacuum to the apparatus in order to keep the pressure low when water vapor is being created. Is there any way to create a vacuum chamber and then ‘cap’ the system so it is self-contained and does not require leaving a vacuum pump running?
    Yes. You can use a glass chamber with a small glass tube opening for the vacuum pump. Then with the pump running, melt the tube to close off the opening. That is how light bulbs are made. a double chamber would be best so you can have two temperatures one for boiling and one for condensing.

    What if the initial pressure was so low that the water vapor created did not increase the pressure significantly?
    If you start with plenty of water, or even full of water, and room temperature or below, then stop when there is 1/3 to 1/4 water, this will not be a problem since PV=nRT, it will seek equilibrium and the pressure will remain at the saturation pressure and temperature until the water is gone.

    What about mechanically expanding or compressing the chamber?
    Fine but it will be hard or impossible to maintain a good seal so you will likely need to keep the vacuum pump connection open

    2. I have read about ‘de-gassing’ a liquid in a vacuum and my understanding of this is when the soluble air particles in the water are pulled out. This creates the appearance of boiling but it is deceptive and only lasts about 60 seconds.
    Right.

    Once this stops, if the temperature is raised and the pressure is lowered will the water ‘boil’ or does degassing prevent boiling from happening?
    The water will boil just fine with or without the air . The air only changes the boiling point slightly due to partial pressure differences.

    I appreciate all of your help on this subject. Chemistry was never my strong point in school and I am attempting to work with what little knowledge I have.
    You seem to be doing fine.


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